In semiconductor device characterization and failure analysis, the mapping of surface topography as well as electrical, optical, and magnetic properties on a submicron scale is increasingly important. As the critical device dimensions shrink to the 0.1 micron level, traditional characterization and failure-analysis methods reach resolution limits. The new methods based upon Scanning-Probe Microscopy (SPM), which can in some cases obtain atomic-scale lateral resolution, are thus becoming more attractive.
SPM comprises a family of microscope designs which share three features. An SPM includes a sharp probe tip which monitors a property of the sample surface lying in the XY plane. An SPM further includes an XYZ piezo drive which can scan the probe tip in the XY plane to image the sample surface, as well as move the probe in the Z direction to allow tracking of the sample surface. The final and perhaps most crucial feature of an SPM is a means for monitoring the tip-sample interaction. Thus, by using control electronics to servo such that the tip-sample interaction is held constant and by scanning the probe tip across the sample in the XY plane, the sample topography and other material properties may be simultaneously mapped on a nanometer scale.
The first SPM was the Scanning Tunneling Microscope (STM), in which the probe tip comprised a sharpened piece of wire and the tip-sample interaction was monitored by measuring the electron tunneling current between the tip and a metallic sample surface. This instrument can obtain atomic-scale maps of the sample surface and has revolutionized surface science but is of limited use for characterization and failure analysis of semiconductor devices due to its requirement of a pristine metallic surface everywhere on the sample; an STM cannot image samples which contain insulating portions.
Hence, it is an object of the present invention to image conducting and non-conducting surfaces.
The Atomic-Force Microscope (AFM) alleviated this constraint by monitoring the force between the probe tip and the sample surface. Other than Atomic-scale surface-science applications, most modern SPM instruments are based upon the AFM. For an AFM, the probe tip is micromachined from Si or Si.sub.3 N.sub.4 and mounted to a miniature cantilever roughly 100-200 microns in length. The deflections and vibrations of this cantilever are monitored by the deflection of a laser beam reflected from the back of the cantilever. Unfortunately, this light illuminates the sample due to the cantilever's partial transparency as well as multiple reflections in the optical system. If the object is to perform electrical measurements on semiconductors, this illumination will cause inaccuracy due to optical carrier generation. Furthermore, for topographic measurements of transparent samples, this illumination can cause laser-induced imaging artifacts.
Hence, it is an object of the present invention to eliminate the laser from the SPM by using a non-optical means for sensing the tip-sample interaction, thus allowing the accurate measurement of electrical properties on light-sensitive materials such as semiconductors and the artifact-free imaging of transparent samples.
Several non-optical means for sensing tip-sample interactions have been proposed. See J. W. P. Hsu, Mark Lee, and B. S. Deaver, REV. SCI. INST. 66 (5), (May 1995)(proposed using a miniature piezoelectric tube, similar to but smaller than that in the XYZ piezo scanner, as an actuator and a sensor of tip oscillations). The piezoelectric tube was driven electrically at one of its mechanical resonances and its electrical impedance was monitored to detect energy dissipation, and hence oscillation-amplitude reductions, due to tip-sample interactions. Unfortunately, even a miniature piezo tube is bulky compared to the nanoscale tip-sample interactions, so the sensitivity of the technique is quite low. Furthermore, the impedance change due to the tip-sample interaction is only a tiny fraction of the total impedance, thus requiring a bridge nulling method of measurement which is very unstable under thermal drift and noise. In addition, materials comprising piezo tubes such as PZT have a low material Q, which results from high intrinsic energy dissipation in the material; this reduces the sensitivity of the detector, which detects energy dissipation in the tip-sample interactions.
Hence, it is an object of the present invention to provide a means of sensing SPM tip-sample interaction which is sensitive enough to detect nanoscale tip-sample interactions. It is a further object of the present invention to provide a means of sensing the SPM tip-sample interaction which does not require nulling methods in its detection and is hence immune from drift. It is a further object of the present invention to provide a means of sensing the tip-sample interaction which employs high-Q materials for all resonant structures.
Another approach to sensing the tip-sample interaction is to use a quartz crystal oscillator, as was suggested by Wolfgang D. Pohl in U.S. Pat. No. 4,851,671. The advantage quartz holds over other piezoelectric materials is its low intrinsic mechanical dissipation, resulting in a potentially high quality factor Q when used properly as an oscillator. Unfortunately, Pohl's sensor used a high-frequency (MHz range), bulk quartz crystal as shown in FIG. 1a. A quartz bulk-crystal oscillator 21 with electrodes 22 and 23 is attached to an XYZ piezo drive 20. A probe tip 24 is mounted to electrode 23. A quartz crystal oscillator of this geometry has a mechanical resonant frequency in the MHz range, and its vibration can be either in the thickness mode or the shear mode. Thus, depending on the quartz polarization, Pohl's microscope will oscillate probe tip 24 either perpendicularly or parallel to a sample surface 25. Because the resonant frequency of quartz bulk-crystal oscillator 21 lies in the MHz range (rather than the kHz range as in the cantilevers of traditional SPM), it is not sensitive to the nanoscale dissipation of tip-sample interactions. Furthermore, the bulk nature of quartz bulk-crystal oscillator 21 introduces a strong coupling between its resonant oscillations and its mounting to XYZ piezo drive 20. Thus, the Q of the oscillator is limited by the high material dissipation of the mounting means rather than the low material dissipation of quartz, further limiting the sensitivity of the tip-sample interaction detection.
Hence, it is a further object of the present invention to provide a means of sensing SPM tip-sample interaction which is immune from dissipative interactions with the mounting means to attach it to the XYZ piezo drive, which would mask the nanoscale dissipation of the tip-sample interactions.
A variant of Pohl's technique uses a quartz tuning-fork oscillator rather than a bulk quartz crystal, thus eliminating the coupling between the resonant vibrations of the quartz oscillator and the mounting means. This is because the tines of a tuning fork vibrate in opposition, and the base of the tuning fork is at a node of oscillation. Because the tuning fork does not cause motion in its base and hence its mounting means, energy is not dissipated in the base or the mounting means, and hence the Q of a quartz oscillator constructed as a tuning fork is limited only by the small material dissipation of quartz, hence Q can assume higher values for a quartz tuning-fork oscillator than for a quartz bulk-crystal oscillator. Klaus Dransfeld, et al., U.S. Pat. No. 5,212,987, discloses an Acoustic Screen Scan Microscope (ASSM) employing the acoustic interaction between a quartz tuning-fork oscillator and the sample surface to detect the tip-sample interactions. In this case, the tip was a corner of one tine of the quartz tuning-fork oscillator. However, the ASSM does not function as an SPM due to the long range of the acoustic interaction of a tip and a sample.
Hence, it is an object of the present invention to provide a means of sensing the tip-sample interaction which is local enough to allow high-resolution, nm-scale imaging.
Recently, Khaled Karrai (patent GB 2,289759 B) has used a quartz tuning-fork oscillator for sensing tip-sample interactions in a Scanning Near-Field Optical Microscopy (NSOM), as shown in FIG. 1b. The reference by Karrai teaches a structure in which the tines of a quartz tuning-fork oscillator 30 are perpendicular to a sample surface 33, hence the probe tip 31 (in Karrai's case a tapered optical fiber) oscillates in a direction along the sample surface. The base of the quartz tuning-fork oscillator 30 is mounted to an XYZ piezo drive 32. Unfortunately, "shearing" the probe tip along the surface is far more damaging to both tip and sample than lightly "tapping" the tip normally on the sample surface. For previous NSOM designs, this is not so critical, since the diameter of tapered optical fiber probes is of the order of 100 nm. However, the technology of Karrai is not appropriate for high-spatial-resolution imaging that other SPM techniques require. For instance, routine AFM analysis can obtain sub-10 nm spatial resolution and some atomic-resolution (i.e., sub-nanometer) work has been done using AFM. Attaining these high-resolution images would be impossible if the tip were sheared across the surface.
It is an object of the present invention to provide a means of sensing the tip-sample interaction which does not damage the tip or the sample by shearing the tip across the sample surface.